Lithium-ion secondary batteries have been widely used in recent years since the lithium-ion secondary batteries are high in energy density and can be charged and discharged. The lithium-ion secondary batteries, which are low in memory effect, are used in, for example, mobile phones, which are frequently charged. Note that the memory effect is a degradation phenomenon as below. If a battery which has not been fully discharged is repeatedly recharged, discharge voltage remarkably declines though the battery has not been fully discharged. As a result, storage capacity of the battery appears to have declined. The term “memory effect” comes from “memorizing” remaining battery level at a time of start of charging.
Broadly speaking, a process of manufacturing a lithium-ion secondary battery includes the following steps: 1. a step of applying a positive-electrode active material to an electrode foil to prepare a positive electrode, and applying a negative-electrode active material to an electrode foil to prepare a negative electrode; 2. a step of drying the positive electrode and the negative electrode which are in roll forms; 3. a step of stacking the positive electrode, the negative electrode, and a separator, which is interposed between the positive electrode and the negative electrode, so as to prepare an electrode pair; 4. a step of accommodating the electrode pair thus prepared in a container and then drying the electrode pair; 5. a step of preparing a cell by injecting an electrolyte solution into the container in which the dried electrode pair is accommodated; and 6. a step of sealing the cell.
In each of the steps 2 and 4, vacuum drying is carried out. The reason why drying is carried out in these two steps is that a large amount of moisture adhered to the positive electrode and the negative electrode cannot be satisfactorily removed by drying in only one of the steps 2 and 4. Residual moisture in the positive electrode and the negative electrode causes, for example, a problem such that the residual moisture mixed into an electrolyte solution results in decomposition of the electrolyte solution, or a problem such that a positive-electrode active material is altered in quality. Therefore, removal of moisture is crucial.
Note here that a technique for removing moisture in a short time is disclosed in Patent Literature 1. Patent Literature 1 relates to a method for manufacturing a battery including a separator which is constituted by an insulating material particulate aggregate layer.